Recently, tremendous research and development effort has been directed to CDMA systems. The CDMA system falls into a direct sequence (DC) system and a frequency hopping (FH) system. The FH system is seldom used at present. This is because the FH system resolves a symbol into elements called chips, and translates individual chips into carriers of different frequencies at a high rate, which is difficult to be implemented by a frequency synthesizer in the state of the art.
Thus, the DS system is commonly used. The DS system performs, at a transmitter, a primary modulation of an original signal use in QPSK or the like, spreading (secondary modulation) of the primary modulated signal into a wideband signal using a spreading code, and transmission thereof on a carrier, and at a receiver, removal of the carrier, despreading (secondary demodulation) of the wideband signal into the primary modulated signal using the spreading code identical to that of the transmitter, and recovery of the original signal by a primary demodulation.
Problems involved in applying the CDMA system to mobile communication systems such as portable telephony will be considered. Base station equipment of mobile communications today includes a highly stable reference oscillator, and the accuracy under the domestic standard for digital vehicle telephony in Japan is less than 0.05 ppm in absolute accuracy. On the other hand, mobile station equipment usually employs a temperature compensated crystal oscillator (TCXO) because of difficulty of using a highly stable reference oscillator with a thermostat. The frequency accuracy of the crystal oscillators is approximately 3 ppm in absolute accuracy in an 800 MHz band.
As a result, the frequency of the local signal of a mobile station will deviate from the center frequency of a transmitted signal of a base station (that is, a received signal of the mobile station). To compensate for the offset of the frequency to maintain stable receiving operation, an AFC (Automatic Frequency Control) circuit is required. In addition, since a mobile station is usually moving with respect to the base station, the center frequency of the received signal further deviates by an amount corresponding to the Doppler frequency.
FIG. 1 shows a major portion of a conventional CDMA receiver with an AFC circuit. An intermediate frequency (IF) received signal applied to an input terminal 10 is divided into two parts by a hybrid coil 10A, and is supplied to a quadrature detector 11. The quadrature detector 11 detects the IF received signal by a local signal from a voltage controlled oscillator (VCO) 12, and outputs an in-phase baseband signal I and a quadrature baseband signal Q. These baseband signals are baseband signals spread by a spreading code. The I baseband signal is supplied to an A/D converter 15 through an LPF (lowpass filter) 13, and is converted into a digital signal. Likewise, the Q baseband signal is supplied to an A/D converter 16 through an LPF 14, and is converted into a digital signal.
These digital signals are fed to a correlator 17 consisting of matched filters or a sliding correlator, which detects correlation between the received digital signals and the spreading code, thereby despreading the digital signals. In other words, the correlator 17 functions as a despreader, and outputs baseband signals corresponding to the primary modulated signal. The outputs of the correlator 17 are supplied to a RAKE receiver and demodulator 18 which demodulates the baseband signals corresponding to the primary modulated signal and recovers the original signal.
An AFC circuit 20 is arranged as follows: The outputs of the correlator 17 are supplied to a differential demodulator 21. Signals undergone the differential demodulation are supplied to a phase error detector 22. The phase error detector 22 obtains a phase error component tan.sup.-1 (Q/I) from the amplitudes of the I and Q signals, and outputs it. This output signal represents a phase rotation component, that is, a phase error at the receiver with respect to a mapped point of the primary modulated signal at the transmitter. In other words, a phase error signal is obtained which is proportional to a frequency error between the received signal and the local signal. The phase error signal is averaged by a loop filter 23, and fed back to the VCO 12 as a control voltage. Thus, the VCO 12 undergoes feedback control by the error correcting voltage corresponding to the phase error signal, so that the frequency error between the center frequency of the received signal and the frequency of the local signal is corrected.
According to this method, a problem arises in that the mobile unit becomes expensive because a highly accurate, highly stable VCO is required as the VCO 12.